The present invention relates generally to an expert detection system for use with an electrical utility power system, and more particularly to an expert detection system for detecting high impedance faults on power lines, such as on a utility's distribution circuit feeder lines.
High impedance, low current faults, such as a broken power line conductor which is touching the ground, are more difficult to detect than permanent overcurrent faults. Conventional overcurrent protection devices have time delays which allow clearing of a temporary fault, such as the momentary contact of a tree branch with a power line. The conventional overcurrent protection devices will only deenergize the power line if the overcurrent fault persists. High impedance, low current faults may initialize the timing circuits of the overcurrent protection devices, but by the end of the delay, the high impedance nature of the fault limits the fault current to a low value. The overcurrent protection devices are unable to distinguish this low fault current from the levels of current ordinarily drawn by customers, so the power line may remain energized even though a conductor has broken. Preferably, this event should trigger "a trip" or opening of a circuit breaker to deenergize the power line.
Unfortunately, distinguishing high impedance faults from the other normal system events and activities is quite complicated. While it is relatively easy to detect the presence of any fault on a distribution feeder, including high impedance faults, it is quite difficult to distinguish high impedance faults from normal system loads and switching operations for a variety of reasons. For example, the current signatures of high impedance faults resemble many normal system events and activities, so the high impedance faults cannot be distinguished by commercially available overcurrent relays. As a further complication, high impedance faults behave differently in different fault situations, such as different seasons and geographic locations. The ability to distinguish high impedance faults from the normal power system events largely determines the relative balance between security and dependability of a distribution feeder protection system.
Over the past few years, several high impedance fault detection techniques have been developed which show somewhat satisfactory performances. The inventor has been involved in various research projects which have yielded several detection methods each having good individual performance. For example, in the inventor's previous U.S. Pat. No. 4,466,071, a high impedance fault detection apparatus and method detect high impedance arcing faults by monitoring the duration of an increase in the magnitude of the high frequency current components having frequencies of 1,000 Hz and above. This detection system uses a microcomputer which performs the monitoring. When a fault occurs, the microcomputer either sets a flag to indicate the occurrence of a fault or trips a circuit breaker to deenergize the faulted power line.
In analyzing the results of these earlier research efforts, it becomes apparent that no single technique or approach offers the sensitivity and discriminatory fault detection required to accurately distinguish a high impedance fault from normal system events and activities. Furthermore, no single detection technique provides a protection system having satisfactory security, dependability and reliability to both protect the public from the dangers of a downed conductor, and to minimize unnecessary power outages from false trips.
Thus, a need exists for an improved expert detection system for detecting high impedance faults on power lines, such as on distribution feeder lines, which is directed toward overcoming, and not susceptible to, the above limitations and disadvantages.